U.S. patent number 7,545,354 [Application Number 11/172,479] was granted by the patent office on 2009-06-09 for driving circuit active matrix type organic light emitting diode device and method thereof.
This patent grant is currently assigned to LG. Display Co., Ltd.. Invention is credited to Yong-Min Ha, Jae-Ho Sim.
United States Patent |
7,545,354 |
Ha , et al. |
June 9, 2009 |
Driving circuit active matrix type organic light emitting diode
device and method thereof
Abstract
Disclosed are a driving circuit and driving method for an
organic light emitting diode (OLED) device. The driving circuit for
the OLED device comprises RGB pixels each including: a gate line
arranged in a first direction and a data line and a power supply
line arranged in a second direction crossing the first direction; a
plurality of switching transistors connected to the region where
the gate line and the data line intersect; a capacitor connected to
the switching transistors and the power supply line; a driving
transistor connected to the capacitor and the power supply line; an
OLED connected to the driving thin film transistor; a variable
voltage signal connected to one of the plurality of switching
transistors; and a driving signal connected to at least one of the
switching transistors, wherein the variable voltage signal is
independently connected to the RGB pixels, and the transistors are
thin film transistors.
Inventors: |
Ha; Yong-Min (Guml,
KR), Sim; Jae-Ho (Daegu, KR) |
Assignee: |
LG. Display Co., Ltd. (Seoul,
KR)
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Family
ID: |
36139527 |
Appl.
No.: |
11/172,479 |
Filed: |
June 30, 2005 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060043366 A1 |
Mar 2, 2006 |
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Foreign Application Priority Data
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Aug 31, 2004 [KR] |
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10-2004-0069348 |
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Current U.S.
Class: |
345/83 |
Current CPC
Class: |
G09G
3/3233 (20130101); G09G 2300/0861 (20130101); G09G
2310/0262 (20130101); G09G 2300/0819 (20130101); G09G
2300/0417 (20130101); G09G 2320/0242 (20130101); G09G
2300/0842 (20130101); G09G 2300/043 (20130101) |
Current International
Class: |
G09G
3/32 (20060101) |
Field of
Search: |
;345/76,82,204,83
;315/169.1,169.3 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
First Notification of Office Action in corresponding Chinese Patent
Application No. 200510082442.6, Jan. 4, 2008. cited by
other.
|
Primary Examiner: Awad; Amr
Assistant Examiner: Sherman; Stephen G
Attorney, Agent or Firm: Brinks Hofer Gilson & Lione
Claims
What is claimed is:
1. A driving circuit for an organic light emitting diode device
comprising: an organic light emitting diode(OLED) formed at each of
a plurality of RGB unit pixels being defined by a gate line
arranged in a first direction and a data line arranged in a second
direction crossing the gate line; a data driving portion for
supplying a data voltage to the RGB unit pixel; a power supply
portion for supplying a high potential voltage to the RGB unit
pixel; a driving thin film transistor having a source terminal
connected to the power supply portion and a drain terminal
connected to the OLED to supply a current to the OLED; a first
switching thin film transistor having a gate terminal connected to
the gate line and a source terminal connected to the data line; a
storage capacitor having terminals connected to the drain terminal
of the first switching transistor and the gate terminal of the
driving thin film transistor; a second switching thin film
transistor having a drain terminal connected to the drain terminal
of the first switching thin film transistor and the one terminal of
the storage capacitor; a third thin film transistor having a source
terminal connected to the power supply portion and a drain terminal
connected to the gate terminal of the driving thin film transistor
and one terminal of the storage capacitor; a variable voltage
portion connected to one side node of the storage capacitor to
which the data driving portion is connected through the second
switching thin film transistor to supply a different voltage to
each of RGB unit pixel; and at least one control signal supply
portion including a first pulse signal portion and a second pulse
signal portion generating and applying a first pulse signal or/and
a second pulse signal to control the first to third switching thin
film transistors, wherein the first to third switching thin film
transistors maintain a data voltage charged to the storage
capacitor by connecting the one side node of the storage capacitor
to the variable voltage portion for supplying the variable voltage
smaller than the data voltage and by floating electrically the
other side node of the storage capacitor, and the first to third
switching thin film transistors control the amount of current of
the driving thin film transistor, wherein the first pulse signal
portion is connected to the first switching thin film transistor
and at least one switching thin film transistor of the second
switching thin film transistor and the third switching thin film
transistor and the second pulse signal portion is connected the
other switching thin film transistor so that a first pulse signal
is supplied to the switching thin film transistors connected to the
first pulse signal portion to turn these on simultaneously and the
second pulse signal is supplied to the switching thin film
transistors connected to the second pulse signal portion to turn
these on simultaneously.
2. The driving circuit of claim 1, further comprising a fourth thin
film transistor having a source terminal connected to the drain
terminal to the driving thin film transistor and a drain terminal
connected to the OLED.
3. The driving circuit of claim 2, wherein the second pulse signal
is supplied to the fourth thin film transistor to turn on
thereof.
4. A method for an organic light emitting diode device, the organic
light emitting diode device including: an organic light emitting
diode(OLED) formed at each of a plurality of RGB unit pixels being
defined by a gate line arranged in a first direction and a data
line arranged in a second direction crossing the gate line; a data
driving portion and a power supply portion; a driving thin film
transistor having a source terminal connected to the power supply
portion and a drain terminal connected to the OLED; a first
switching thin film transistor having a gate terminal connected to
the gate line and a source terminal connected to the data line; a
storage capacitor having terminals connected to the drain terminal
of the first switching transistor and the gate terminal of the
driving thin film transistor; a second switching thin film
transistor having a drain terminal connected to the drain terminal
of the first switching thin film transistor and the one terminal of
the storage capacitor; a third thin film transistor having a source
terminal connected to the power supply portion and a drain terminal
connected to the gate terminal of the driving thin film transistor
and one terminal of the storage capacitor; a variable voltage
portion connected to one side node of the storage capacitor to
which the data driving portion is connected through the second
switching thin film transistor to supply a different voltage to
each of RGB unit pixel; and at least one control signal supply
portion including a first pulse signal portion and a second pulse
signal portion generating and applying a first pulse signal and/or
a second pulse signal to control the first to third switching thin
film transistors, the method comprising: supplying the high
potential voltage to the RGB unit pixel; and supplying a first
pulse signal to the switching thin film transistors connected to
the first pulse signal portion to turn these on simultaneously and
supplying the second pulse signal to the switching thin film
transistors connected to the second pulse signal portion to turn
these on simultaneously, whereby the one side node of the storage
capacitor is connected to the variable voltage portion for
supplying the variable voltage smaller than the data voltage and
other side node of the storage capacitor is floating electrically
to maintain the data voltage charged to the storage capacitor.
5. The method of claim 4, wherein the organic light emitting diode
device including a fourth thin film transistor having a source
terminal connected to the drain terminal to the driving thin film
transistor and a drain terminal connected to the OLED.
6. The method of claim 5, wherein supplying the second pulse signal
including supplying the second pulse signal is supplied to the
fourth thin film transistor.
Description
PRIORITY CLAIM
This application claims the benefit of the Korean Patent
Application No. 69348/2004, filed on Aug. 31, 2004, which is hereby
incorporated by reference.
FIELD OF THE INVENTION
The present invention relates to a driving circuit of an active
matrix type organic light emitting diode device, and more
particularly to, a driving circuit and driving method for an active
matrix type organic light emitting diode device, which can improve
luminance uniformity between panels by compensating for changes in
threshold voltage of a polycrystalline silicon thin film transistor
existing between organic light emitting diode devices.
DESCRIPTION OF THE BACKGROUND ART
In recent years, liquid crystal devices (LCDs) are currently most
commonly used as a flat panel display (FPD) due to the advantage of
light weight and low power consumption.
However, the liquid crystal devices are not a self light emitting
element but a light receiving element and have technical
restrictions in brightness, contrast, viewing angles, large size,
etc. Thus, recently, the efforts to develop new flat panel displays
for overcoming such disadvantages have been actively pursued.
An organic light emitting diode, one of the new flat panel
displays, is superior to a liquid crystal display in viewing
angles, contrast, etc. because it is a self light emitting type,
and can be made lightweight and thin, and is advantageous from a
power consumption point of view because it requires no
backlight.
Additionally, the organic light emitting diode has an advantage
that it is strong to an external shock, provides a wide range of
temperature because it is capable of direct current low voltage
driving, has a fast response speed, and is made entirely in a solid
phase. Furthermore, it has a cheap manufacturing cost.
In a manufacturing process of the organic light emitting diode
device, all that is needed is deposition and encapsulation
equipment unlike a liquid crystal device or PDP (plasma display
panel), thus the process is very simple.
If the organic light emitting diode device is driven in an active
matrix type having thin film transistors, which are switching
devices for each pixel, it shows the same luminance even if a low
current is applied. This enables low power consumption, high
definition, and large size.
FIG. 1 is a view showing a basic structure of a general active
matrix type organic light emitting diode device (AMOLED). In FIG.
1, the general organic light emitting diode display panel comprises
gate lines GL1.about.GLm and data lines DL1.about.DLn arranged to
cross each other on a glass substrate with pixel portions 30 formed
respectively in rectangular regions of a matrix pattern defined by
the gate lines GL1.about.GLm and the data lines DL1.about.DLn
crossing each other.
The pixel portions 30 are driven in units of gate lines
GL1.about.GLm by a scanning signal applied via the gate lines
GL1.about.GLm, and generates light corresponding to the intensity
of image signals applied via the data lines DL1.about.DLn.
Therefore, in the organic light emitting diode display panel, a
scanning line driving circuit 10 for applying scanning signals to
the gate lines GL1.about.GLm and a data driving circuit for
supplying image signals to the data lines DL1.about.DLn are
manufactured on a single crystal silicon substrate, and attached on
a glass substrate of the organic light emitting diode display panel
in the same method as a taper carrier package (TCP).
In the image display portion, a plurality of gate lines
GL1.about.GLm arranged in a transverse direction at regular
intervals and a plurality of data lines DL1.about.DLn arranged in a
column direction at regular intervals cross each other. In the
regions defined by the gate lines GL1.about.GLm and the data lines
DL1.about.DLn crossing each other, pixels 100 electrically
connected to the gate lines GL1.about.GLm and the data lines
DL1.about.DLn are respectively provided.
The pixels 100 are driven in units of gate lines GL1.about.GLm by a
scanning signal applied via the gate lines GL1.about.GLm, and
generates light corresponding to the intensity of image signals
applied via the data lines DL1.about.DLn.
FIG. 2 is a circuit diagram showing a unit pixel of a general
active matrix type organic light emitting diode device. In FIG. 2,
a gate line GL is formed in a first direction, and a data line DL
and a power supply line V.sub.DD formed at a given interval in a
second direction crossing the first direction, thereby forming one
pixel region.
A switching thin film transistor TR2, an addressing element, is
connected to the region where the gate line GL and the data line DL
intersect. A storage capacitor (hereinafter, referred to as Cst) is
connected to the switching thin film transistor TR2 and the power
supply line V.sub.DD. A driving thin film transistor TR1, a current
source element, is connected to the storage capacitor Cst and the
power supply line V.sub.DD, and an electroluminescent diode EL is
connected to the driving thin film transistor TR1.
The switching thin film transistor TR2 includes a source electrode
S1 connected to the gate line GL and supplying a data signal and a
drain electrode D1 connected to a gate electrode G2 of the driving
thin film transistor TR1, and which switches the electroluminescent
diode EL.
The driving thin film transistor TR1 includes a gate electrode G2
connected to the drain electrode D1 of the switching thin film
transistor TR2, a drain electrode connected to an anode electrode
of the electroluminescent diode EL and a source electrode S2
connected to the power line V.sub.DD, and serves as a driving
device of the electroluminescence diode.
In the storage capacitor Cst, an electrode at one side is commonly
connected to the drain electrode D1 of the switching thin film
transistor TR2 and the gate electrode of the driving thin film
transistor TR1, and an electrode at the other side is connected to
the source electrode S2 and of the driving thin film transistor and
the power line V.sub.DD.
The electroluminescence diode EL includes an anode electrode
connected to the drain electrode D2 of the driving thin film
transistor TR1, a cathode electrode connected to the ground line
GND and an organic light emitting layer formed between the cathode
electrode and the anode electrode. The organic light emitting layer
is comprised of a hole carrier layer, a light emitting layer and an
electron carrier layer.
The thus-constructed general organic light emitting diode device
(AMOLED) supplies currents through the thin film transistors.
Because conventional amorphous silicon thin film transistors are
low in carrier mobility, polysilicon thin film transistors with
improved carrier mobility have been employed in recent years.
In order to show a minute color change, a good gray scale
capability is a must-have function in displays.
The aforementioned organic light emitting diode device displays
images by controlling the amount of current flowing in the
electroluminescence diode. The organic light emitting diode device
displays gray scales by differentiating the amount of light
emission of the organic light emitting diode device by controlling
the amount of current flowing in the thin film transistors for
supplying currents to the organic light emitting diode device in an
active driving method.
However, according to a driving circuit and driving method of an
organic electroluminescence display device according to the
conventional art, the current of the organic light emitting diode
is determined according to a gate voltage V.sub.IN of a driving
polycrystalline silicon thin film transistor TR1.
The driving polycrystalline silicon thin film transistor TR1
operates in a saturation region, thus a flowing current is
expressed by the following formula (1): I.sub.DS=W/L .mu.p C.sub.OX
(V.sub.DD-V.sub.IN+V.sub.TH).sup.2 (1)
wherein W denotes a channel width of the driving thin film
transistor, L denotes a channel length, .mu.p denotes a charge
transfer rate, V.sub.DD denotes a power supply line, V.sub.IN
denotes a gate voltage, and V.sub.TH denotes a threshold
voltage.
If the threshold voltage of the driving polycrystalline silicon
thin film transistor TR1 between panels is changed, the current of
the driving polycrystalline silicon thin film transistor TR1 and
the current of the organic light emitting diode are also changed,
thereby making the luminance between panels non-uniform.
SUMMARY OF THE INVENTION
A driving circuit and driving method for an active matrix type
organic light emitting diode device, which can improve luminance
uniformity between panels by compensating for changes in threshold
voltage of a polycrystalline silicon thin film transistor existing
between organic light emitting diode devices.
Additionally, a driving circuit and driving method for an active
matrix type organic light emitting diode device, may reduce power
consumption by gamma compensation by changing a variable voltage
Vref value and compensate for the non-uniformity of the
characteristics of RGB organic light emitting diodes by applying a
variable voltage Vref for each RGB pixel.
A driving circuit for an organic light emitting diode device may
comprise a plurality of RGB pixels each including: a gate line
arranged in a first direction, a data line arranged in a second
direction crossing the gate line, and a power supply line arranged
in the second direction, at a given interval from the data line,
crossing the gate line; a plurality of switching thin film
transistors connected to the region where the gate line and the
data line intersect; a storage capacitor coupled to at least one of
the switching thin film transistors and the power supply line; a
driving thin film transistor connected to the storage capacitor and
the power supply line; an organic light emitting diode coupled to
the driving thin film transistor; a variable voltage signal
connected to one of the plurality of switching thin film
transistors; and a SELECT signal connected to at least one of the
plurality of switching thin film transistors, wherein the variable
voltage signal is independently connected to the each of the RGB
pixels.
Each of the RGB pixels comprises: a first switching thin film
transistor connected to the data line; a storage capacitor
connected to the first switching thin film transistor; a driving
thin film transistor connected to the storage capacitor and the
power supply line; and a second switching thin film transistor
connected to the driving thin film transistor.
The driving circuit of the organic light emitting diode device may
comprise: a third switching thin film transistor connected to the
second switching thin film transistor connected between the first
switching thin film transistor and the storage capacitor to be
coupled to the variable voltage signal; and a fourth switching thin
film transistor connected to the storage capacitor and between a
gate and a drain of the driving thin film transistor, coupled to
the first switching thin film transistor and connected to the
SELECT signal.
The second switching thin film transistor and the third switching
thin film transistor may be coupled to the EM signal.
In the driving circuit for the organic light emitting diode device,
each of the RGB pixels may comprise: a first switching thin film
transistor connected to the data line and coupled to the SELECT
signal; a second switching thin film transistor connected between
the first switching thin film transistor and the storage capacitor
and coupled to the variable voltage signal; and a third switching
thin film transistor connected to the storage capacitor and between
a gate and a drain of the driving thin film transistor.
The gate of the second switching thin film transistor may be
coupled to the EM signal.
In the driving circuit for the organic light emitting diode device,
each of the RGB pixels may comprise: a first switching thin film
transistor connected to the data line and coupled to the SELECT
signal; a second switching thin film transistor connected between
the first switching thin film transistor and the storage capacitor
and coupled to the variable voltage signal; and a third switching
thin film transistor connected to the storage capacitor and between
a gate and a drain of the driving thin film transistor.
The gate of the second switching thin film transistor may be
coupled to the SELECT signal.
There is provided a method of driving an organic light emitting
diode device according to the invention, wherein a plurality of RGB
pixels are driven by: arranging a gate line in a first direction;
arranging a data line in a second direction crossing the gate line;
arranging a power supply line in the second direction, at a given
interval from the data line, crossing the gate line; connecting a
plurality of switching thin film transistors to a region where the
gate line and the data line intersect; connecting a storage
capacitor to the switching thin film transistors and the power
supply line; connecting a driving thin film transistor to the
storage capacitor and the power supply line; connecting an organic
light emitting diode to the driving thin film transistor;
connecting a variable voltage signal to one of the plurality of
switching thin film transistors; and connecting a SELECT signal
connected to at least one of the plurality of switching thin film
transistors, wherein the variable voltage signal is independently
connected to the RGB pixels and a variable voltage used for
preserving a data voltage stored in the respective storage
capacitors of the RGB pixels for one frame to adjust the current
value of the respective organic light emitting diodes of the RGB
pixels.
Other systems, methods, features and advantages of the invention
will be, or will become, apparent to one with skill in the art upon
examination of the following figures and detailed description. It
is intended that all such additional systems, methods, features and
advantages be included within this description, be within the scope
of the invention, and be protected by the following claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are included to provide a further
understanding of the invention and are incorporated in and
constitute a part of this specification, illustrate embodiments of
the invention and together with the description serve to explain
the principles of the invention.
In the drawings:
FIG. 1 is a view showing a basic structure of a general active
matrix type organic light emitting diode device (AMOLED);
FIG. 2 is a circuit diagram showing a unit pixel of a general
active matrix type organic light emitting diode device;
FIG. 3 is a circuit block diagram showing a unit pixel of an
organic light emitting diode device according to a first embodiment
of the present invention;
FIG. 4 is an exemplary view showing the organic light emitting
diode device to which a Vref voltage for each RGB pixel is applied
according to the first embodiment of the present invention;
FIG. 5 is a circuit block diagram showing a unit pixel of an
organic light emitting diode device according to a second
embodiment of the present invention, in which Vref is used in order
to preserve information stored in Cst for one frame like in the
first embodiment of the present invention;
FIG. 6 is an exemplary view showing the organic light emitting
diode device to which a Vref voltage for each RGB pixel is applied
according to the second embodiment of the present invention;
FIG. 7 is a circuit block diagram showing a unit pixel of an
organic light emitting diode device according to a third embodiment
of the present invention, which illustrates a case where there is
no need to use an EM signal because a n type p-Si TFT is used as
the third switching thin film transistor T4 in the second
embodiment of the present invention; and
FIG. 8 is an exemplary view showing the organic light emitting
diode device to which a Vref voltage for each RGB pixel is applied
according to the third embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 3 is a circuit block diagram showing a unit pixel of an
organic light emitting diode device according to a first embodiment
of the present invention. FIG. 4 is an exemplary view showing the
organic light emitting diode device to which a Vref voltage for
each RGB pixel is applied according to the first embodiment of the
present invention.
In the organic light emitting diode device according to the first
embodiment of the invention, a gate line (not shown) is formed in a
first direction, and a data line (not shown) and a power supply
line V.sub.DD formed at a given interval in a second direction
cross the first direction, thereby forming a pixel region.
A first switching thin film transistor T2, an addressing element,
is connected within a pixel region. A storage capacitor
(hereinafter, referred to as Cst) is connected to the first
switching thin film transistor T2 and the power supply line
V.sub.DD, via transistor T4. A driving thin film transistor T1, a
current source element, is connected to the storage capacitor Cst
and the power supply line V.sub.DD, and an organic light emitting
diode OLED is connected to the driving thin film transistor T1.
A second switching thin film transistor T3 is connected between the
first switching thin film transistor T2 and the storage capacitor
Cst; a third switching thin film transistor T4 is connected between
the gate and drain of the driving thin film transistor T1, and is
connected to the storage capacitor Cst; and a fourth switching thin
film transistor T5 is connected between the driving thin film
transistor T1 and the organic light emitting diode OLED.
The gate of the third switching thin film transistor T4 is
connected to the first switching thin film transistor T2 to be
coupled to a SELECT (n) signal.
The gate of the second switching thin film transistors T3 is
connected to the gate of the fourth switching thin film transistor
T5 to be coupled to an EM (n) signal.
The source of the second switching thin film transistor T3 is
connected to a variable voltage Vref, which is a DC voltage.
The thus constructed driving circuit and driving method for an
organic light emitting diode device according to the first
embodiment of the present invention will be described with
reference to FIGS. 3 and 4.
In FIG. 3, the first and third switching thin film transistors T2
and T4 are turned ON at the section C where the SELECT (n) is
turned ON.
At this time, an A node voltage is initialized to a
V.sub.DD-|V.sub.TH|, and a B node voltage becomes V.sub.DATA.
The second switching thin film transistor T3 is turned ON at the
section D where the SELECT (n) is turned OFF and the EM (n) is
turned ON, whereby the B node voltage becomes a variable voltage
Vref, which is a DC voltage.
The A node voltage is boostrapped by the change rate
(V.sub.DATA-Vref) of the B node voltage, and becomes
"V.sub.DD-|V.sub.TH|-V.sub.DATA-Vref".
In summary of this result, the current of the driving thin film
transistor T1 may be shown as the following expression (2):
I.sub.OLED=1/2 K(|V.sub.GS|-|V.sub.TH|).sup.2=1/2
K(V.sub.DD-V.sub.DD+|V.sub.TH|+V.sub.DATA-Vref-|V.sub.TH|).sup.2=1/2
K(V.sub.DATA-Vref).sup.2 (2)
Wherein K is u.times.Cox.times.W/L
Resultantly, the current I.sub.OLED becomes a function of
V.sub.DATA and Vref
The I.sub.OLED value can be adjusted by adjusting the variable
voltage Vref, which is a DC voltage used for preserving a data
voltage stored in the storage capacitor Cst for one frame.
As shown in FIG. 4, chromaticity and gamma values can be adjusted
by such a circuit construction where the variable voltage Vref
supply signals are disposed for each RGB pixel configured by the
circuit construction as shown in FIG. 3.
It is easier to compensate for the non-uniformity of the
characteristics of the RGB organic light emitting diodes (OLED1,
OLED2, OLED3) by applying a Vref when no current flows through
driving transistor T1 as compared to a conventional structure where
V.sub.DD is applied and the current flowing through driving
transistor T1 is adjusted.
A driving circuit for an organic light emitting diode according to
a second embodiment will be described with reference to the
accompanying drawings.
FIG. 5 is a circuit block diagram showing a unit pixel of an
organic light emitting diode device according to a second
embodiment of the present invention, in which Vref is used in order
to preserve information stored in Cst for one frame like in the
first embodiment of the present invention.
FIG. 6 is an exemplary view showing the organic light emitting
diode device to which a Vref voltage for each RGB pixel is applied
according to the second embodiment of the present invention.
In the organic light emitting diode device according to the second
embodiment of the invention, a gate line (not shown) is formed in a
first direction, and a data line (not shown) and a power supply
line V.sub.DD formed at a given interval in a second direction
crossing the first direction, thereby forming one pixel region.
A second switching thin film transistor T3, an addressing element,
is connected within a pixel region. A storage capacitor
(hereinafter, referred to as Cst) is connected to the second
switching thin film transistor T3 and the power supply line
V.sub.DD. A driving thin film transistor T1, a current source
element, is connected to the storage capacitor Cst and the power
supply line V.sub.DD, and an organic light emitting diode OLED is
connected to the driving thin film transistor T1.
A third switching thin film transistor T4 is connected between the
second switching thin film transistor T3 and the storage capacitor
Cst, and a first switching thin film transistor T2 is connected
between the gate of the driving thin film transistor T1 connected
to the storage capacitor Cst and the power supply line V.sub.DD,
thus coupling the gate to a SELECT (n) signal.
The third switching thin film transistor T4 is connected between
the second switching thin film transistor T3 and the storage
capacitor Cst, thus coupling the source thereof to a variable
voltage Vref, which is a DC voltage. The gate of the second
switching thin film transistor T3 is connected to the SELECT (n)
signal like the first switching thin film transistor T2. Further,
the gate of the third switching thin film transistor T4 is
connected to an EM (n) signal.
In FIG. 5, the first and third switching thin film transistors T2
and T3 are turned ON at the section C where the SELECT (n) signal
is turned ON. At this time, an A node voltage is initialized to a
V.sub.DD and a B node voltage becomes V.sub.DATA.
The second switching thin film transistor T3 is turned ON at the
section D where the SELECT (n) signal is turned OFF and the EM (n)
signal is turned ON, whereby the B node voltage becomes a Vref
voltage.
At this time, the A node voltage is boostrapped by the change rate
(V.sub.DATA-Vref) of the B node voltage, and becomes
"V.sub.DD-|V.sub.TH|-V.sub.DATA-Vref".
In summary of this result, the current of the driving thin film
transistor T1 will be shown as the following expression (2):
I.sub.OLED=1/2 K(|V.sub.GS|-|V.sub.TH|).sup.2=1/2
K(V.sub.DD-V.sub.DD+V.sub.DATA-Vref-|V.sub.TH|).sup.2=1/2
K(V.sub.DATA-Vref-|V.sub.TH|).sup.2 (2)
Wherein K is u.times.Cox.times.W/L
Based on the result of the expression of the current, the current
I.sub.OLED is proportional to a variable voltage Vref as in the
first embodiment, and a uniform luminance between panels can be
obtained by adjusting the variable voltage Vref
As shown in FIG. 6, chromaticity and gamma values may be adjusted
by such a circuit construction that the respective variable voltage
Vref supply signals are connected for each RGB pixel configured by
the circuit construction as shown in FIG. 5.
FIG. 7 is a circuit block diagram showing a unit pixel of an
organic light emitting diode device according to a third embodiment
of the invention, which illustrates a case where there is no need
to use an EM signal because a n type p-Si TFT is used as the third
switching thin film transistor T4 in the second embodiment of the
invention.
FIG. 8 is an exemplary view showing the organic light emitting
diode device to which a Vref voltage for each RGB pixel is applied
according to the third embodiment of the invention.
In the organic light emitting diode device according to the third
embodiment of the invention, a gate line (not shown) is formed in a
first direction, and a data line (not shown) and a power supply
line V.sub.DD formed at a given interval in a second direction
crossing the first direction, thereby forming one pixel region.
A second switching thin film transistor T3, an addressing element,
is connected within a pixel region. A storage capacitor
(hereinafter, referred to as Cst) is connected to the second
switching thin film transistor T3 and the power supply line
V.sub.DD. A driving thin film transistor T1, a current source
element, is connected to the storage capacitor Cst and the power
supply line V.sub.DD, and an organic light emitting diode OLED is
connected to the driving thin film transistor T1.
A third switching thin film transistor T4 is connected between the
second switching thin film transistor T3 and the storage capacitor
Cst, and a first switching thin film transistor T2 is connected
between the gate of the driving thin film transistor T1 connected
to the storage capacitor Cst and the power supply line V.sub.DD,
thus coupling to a SELECT (n) signal.
The third switching thin film transistor T4 is connected between
the second switching thin film transistor T3 and the storage
capacitor Cst, thus coupling to a variable voltage Vreft, which is
a DC voltage. The gate of the second switching thin film transistor
T3 and the gate of the third switching thin film transistor T4 are
connected to the SELECT (n) signal like the first switching thin
film transistor T2.
In FIG. 7, the first and third switching thin film transistors T2
and T3 are turned ON at the section C where the SELECT (n) signal
becomes a low value.
When the SELECT (n) signal is changed from a low value to a high
value, the second switching thin film transistor T3 is turned OFF
and the third switching thin film transistor T4 is turned ON,
whereby the B node voltage becomes a Vref voltage.
At this time, the A node voltage is boostrapped by the change rate
(V.sub.DATA-Vref) of the B node voltage, and becomes
"V.sub.DD-|V.sub.TH|-V.sub.DATA-Vref".
In summary of this result, the current of the driving thin film
transistor T1 will be shown as the following expression (3):
.times..function..times..function..times..function.
##EQU00001##
Wherein K is u.times.Cox.times.W/L
Based on the result of the expression of the current, the current
I.sub.OLED is proportional to a variable voltage Vref as in the
second embodiment, and a uniform luminance between panels can be
obtained by adjusting the variable voltage Vref.
Besides, chromaticity and gamma values can be adjusted by such a
circuit construction that respective variable voltage Vref supply
portions are connected for each RGB pixel.
It is easier to compensate for the non-uniformity of the
characteristics of the RGB organic light emitting diodes (OLED1,
OLED2, OLED3) by applying a Vref when no current flows through
driving thin film transistor T1 as compared to a conventional
structure where V.sub.DD is applied and the current flowing through
driving thin film transistor T1 is adjusted.
While various embodiments of the invention have been described, it
will be apparent to those of ordinary skill in the art that many
more embodiments and implementations are possible within the scope
of the invention. Accordingly, the invention is not to be
restricted except in light of the attached claims and their
equivalents.
* * * * *